The present invention relates to sensors, particularly low .mu.G accelerometer sensors, and more particularly to micromachined rocking accelerometers with capacitive pickoff having low frequency (&lt;500 Hz), high sensitivity (.mu.G), and low power consumption and cost.
Sensors of various types are being utilized for security and environmental monitoring needs. Network of sensors are being utilized for improved range, sensitivity, directionality, and data readability. It is desirable to use sensor networks composed of individual sensors and associated electronics which are small in size and low in power consumption so as to be powered by a battery that is both small and with a long life. Smaller, low power sensors allow the use of more sensors per network, thus increasing the sensitivity of the network.
As pointed out above, a variety of sensors have been developed for various applications. However, because of low power and small size requirements or certain applications, sensors such as fiber optic devices which require laser power or sensor technologies that require heaters cannot be utilized. Recently micromachined accelerometer type sensors have been developed, but these sensors typically have mG sensitivities, and due to the stiffness of support structures, are designed to operate at high frequency (&gt;5 kHz), and thus are not applicable for applications requiring both high sensitivity and low frequency operation, such as for monitoring large scale structures, to detect ground motion, or passing vehicles. The prior micromachined accelerometers are exemplified by U.S. Pat. No. 4,928,203; No. 5,337,606; No. 5,456,111; and No. 5,659,195, and rely on relatively small mass surface techniques, or wafer bonding to produce a larger proof mass; and use an electrode moved in a semi-linear fashion with respect to another electrode.
The present invention satisfies the above-mentioned requirements for small, low power, low cost sensors, but which operates at low frequency (&lt;500 Hz) and has high sensitivity (.mu.G). The present invention involves a micromachined low frequency rocking accelerometer with capacitive pickoff fabricated by deep reactive ion etching, and involves a central silicon proof mass suspended by a thin (2 .mu.m), surface micromachined polysilicon tether. During movement of the sensor, the tethered mass moves relative to the surrounding packaging, and this deflection is measured capacitively by means of plate capacitors or interdigitated finger capacitors. Also, the accelerometer may include a feedback control which enables a greater dynamic range.